Sport boats or jet bike types of watercraft have become increasingly popular. Such craft are capable of planing, skipping, and jumping at speeds up to 40 miles per hour. Because of such speed and increasingly so where waves are generally present, the bottom hull of the craft receives a great deal of punishment due to the pounding against the water at such speeds.
By and large, major sport boat type of watercraft hulls are formed in a closed mold resin transfer molding (RTM) process or a vacuum molding process. In a RTM process, a two-piece mold consists of a female dye (cavity) and a male dye (core) used in the process. A composite mixture of resin, reinforcement, filler, and additives is placed between the two heated matched molds which are then closed under pressure to cure the composite into a solid reinforced plastic part. The reinforcement is usually placed on the lower mold, the mold halves are clamped together, and resin is pumped in under pressure. The most common reinforcement used is glass which is available as fibers, flakes, or spheres. Some carbon fibers and synthetic fibers, alone or combined with glass are also increasing in use. Both thermoplastic and thermoset resins are used in closed mold processing with polyesters being the most popular due to their unique blend of mechanical, electrical, and chemical-resistant properties and cost-effective performance. Thermoplastic resins used are nylon, polypropylene, and thermoplastic polyester. Hulls made by the RTM process are more costly and do not provide satisfactory means to rigidly fasten the propulsion system, hull fittings, hardware, etc. sufficient to take the pounding produced when in use and in many cases results in a flexible bottom hull which contributes to cyclical bouncing against the water.
Another popular form of manufacturing jet type watercraft hulls is by the open mold fiber reinforced plastic (FRP) process. There are three basic materials used in open mold processing: Resins (most frequently polyesters); glass fiber reinforcement in such configurations as woven roving, mat, and gun roving; and catalysts, most frequently methyl ethyl ketone peroxide (MEKP) or Benzoyl peroxide. In a well-known technique, the catalyst initiates a chemical reaction that transforms the liquid into a solid. The glass fiber reinforcement is encapsulated within the resin, giving structural strength. The three materials are applied to the mold simultaneously, either manually or by special spray-up equipment. A chemical chain reaction occurs forming a structural laminate. The laminate after a short carrying time is demolded, producing an exact duplicate of the mold. In a preliminary step, specially pigmented polyester resin (gel-coat) is applied to provide smoothness and color to the product. The gel-coat/catalyst mixture is built up to a thickness of 15 to 20 mil. Both the aesthetic (gel-coat) and structural (laminate) processes are performed in the open mold. Being from the same chemical family, the gel-coat crosslinks with the laminate. Watercraft hulls made by the FRP open mold process are several times less expensive and faster to make than hulls made by closed mold RTM process. However, the resulting hull has many disadvantages, primarily structural loss of strength, rigidity, and stiffness sufficient to withstand the pounding of the craft in use on the water. As in a RTM process, to build up the laminate to reduce this defect adds tremendous weight to the craft, reducing its speed and portability. Another disadvantage of a craft formed by the FRP open mold process is that it produces a rather rough inner surface which is an undesirable characteristic in the interior of the hull since it allows the buildup of fuel residue and oil that cannot be easily cleaned if the surface were not gel-coat smooth.
In addition, flotation is usually required for the hull whether it be a RTM or FRP type. To do so, especially in a FRP type of hull, foam is usually formed in the sides of the craft. This requires a separate step of building a form (usually wood) to form cavities into which polyurethane foam is injected. However, once the foam has set, the release of the mold form is difficult due to the fact that without some form of release agent and in many instances with a release agent, the polyurethane foam tends to adhere to the form, leaving another rough, unsightly, and hard-to-clean interior surface. Polyurethane foams (PUR) generally are formed by mixing two primary ingredients during processing. For most commonly used PURs, the two ingredients are a polyisocyanate and a polyol. Typically, a closed cell PUR foam is formed by the release of a blowing agent which is largely retained as a closed cell.